Low power magnetoresistive random access memory elements

ABSTRACT

Low power magnetoresistive random access memory elements and methods for fabricating the same are provided. In one embodiment, a magnetoresistive random access device has an array of memory elements. Each element comprises a fixed magnetic portion, a tunnel barrier portion, and a free SAF structure. The array has a finite magnetic field programming window Hwin represented by the equation H&lt;SUB&gt;win&lt;/SUB&gt;≈(&lt;img id=&#34;custom-character-00001&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00900.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;Hsat&lt;img id=&#34;custom-character-00002&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00901.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;-&lt;img id=&#34;custom-character-00003&#34; he=&#34;3.13mm&#34; wi=&#34;2.79mm&#34; file=&#34;US20070037299A1-20070215-P00902.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;sigma&lt;SUB&gt;sat&lt;/SUB&gt;)-(&lt;img id=&#34;custom-character-00004&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00900.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;Hsw&lt;img id=&#34;custom-character-00005&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00901.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;+&lt;img id=&#34;custom-character-00006&#34; he=&#34;3.13mm&#34; wi=&#34;2.79mm&#34; file=&#34;US20070037299A1-20070215-P00902.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;sigma&lt;SUB&gt;sw&lt;/SUB&gt;), where &lt;img id=&#34;custom-character-00007&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00900.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;Hsw&lt;img id=&#34;custom-character-00008&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00901.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt; is a mean switching field for the array, &lt;img id=&#34;custom-character-00009&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00900.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt;Hsat&lt;img id=&#34;custom-character-00010&#34; he=&#34;3.13mm&#34; wi=&#34;1.02mm&#34; file=&#34;US20070037299A1-20070215-P00901.TIF&#34; alt=&#34;custom character&#34; img-content=&#34;character&#34; img-format=&#34;tif&#34;/&gt; is a mean saturation field for the array, and Hsw for each memory element is represented by the equation H&lt;SUB&gt;SW&lt;/SUB&gt;≅√{square root over (H&lt;SUB&gt;k&lt;/SUB&gt;H&lt;SUB&gt;SAT&lt;/SUB&gt;)}, where H&lt;SUB&gt;k &lt;/SUB&gt;represents a total anisotropy and H&lt;SUB&gt;SAT &lt;/SUB&gt;represents an anti-ferromagnetic coupling saturation field for the free SAF structure of each memory element. N is an integer greater than or equal to 1. H&lt;SUB&gt;k&lt;/SUB&gt;, H&lt;SUB&gt;SAT&lt;/SUB&gt;, and N for each memory element are selected such that the array requires current to operate that is below a predetermined current value.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 10/997,118 filed on Nov.24, 2004.

FIELD OF THE INVENTION

The present invention generally relates to magnetoelectronic devices,and more particularly relates to magnetoresistive random access memoryelements that require low power for operation.

BACKGROUND OF THE INVENTION

Magnetoelectronic devices, spin electronic devices, and spintronicdevices are synonymous terms for devices that make use of effectspredominantly caused by electron spin. Magnetoelectronics is used innumerous information devices, and provides non-volatile, reliable,radiation resistant, and high-density data storage and retrieval. Thenumerous magnetoelectronics information devices include, but are notlimited to, Magnetoresistive Random Access Memory (MRAM), magneticsensors, and read/write heads for disk drives.

Typically, a magnetoelectronic information device, such as an MRAM,includes an array of memory elements. Each memory element typically hasa structure that includes multiple magnetic layers separated by variousnon-magnetic layers. Information is stored as directions ofmagnetization vectors in the magnetic layers. Magnetic vectors in onemagnetic layer are magnetically fixed or pinned, while the magnetizationdirection of another magnetic layer may be free to switch between thesame and opposite directions that are called “parallel” and“antiparallel” states, respectively. Corresponding to the parallel andantiparallel magnetic states, the magnetic memory element has low andhigh electrical resistance states, respectively. Accordingly, adetection of change in the measured resistance allows amagnetoelectronics information device, such as an MRAM device, toprovide information stored in the magnetic memory element.

FIG. 1 illustrates a conventional memory element array 10 having one ormore memory elements 12. An example of one type of magnetic memoryelement, a magnetic tunnel junction (MTJ) element, comprises a fixedferromagnetic layer 14 that has a magnetization direction fixed withrespect to an external magnetic field and a free ferromagnetic layer 16that has a magnetization direction that is free to rotate with theexternal magnetic field. The fixed layer and free layer are separated byan insulating tunnel barrier layer 18. The resistance of memory element12 relies upon the phenomenon of spin-polarized electron tunnelingthrough the tunnel barrier layer between the free and fixedferromagnetic layers. The tunneling phenomenon is electron spindependent, making the electrical response of the MTJ element a functionof the relative orientations and spin polarization of the conductionelectrons between the free and fixed ferromagnetic layer.

The memory element array 10 includes conductors 20, also referred to asdigit lines 20, extending along rows of memory elements 12 andconductors 22, also referred to as word or bit lines 22, extending alongcolumns of the memory elements 12. A memory element 12 is located at across point of a digit line 20 and a bit line 22. The magnetizationdirection of the free layer 16 of a memory element 12 is switched bysupplying currents to digit line 20 and bit line 22. The currents createmagnetic fields that switch the magnetization orientation of theselected memory element from parallel to anti-parallel, or vice versa.

FIG. 2 illustrates the fields generated by a conventional linear digitline 20 and bit line 22. To simplify the description of MRAM device 10,all directions will be referenced to an x- and y-coordinate system 50 asshown. A bit current IB 30 is defined as being positive if flowing in apositive x-direction and a digit current ID 34 is defined as beingpositive if flowing in a positive y-direction. A positive bit current IB30 passing through bit line 22 results in a circumferential bit magneticfield, HB 32, and a positive digit current ID 34 will induce acircumferential digit magnetic field HD 36. The magnetic fields HB 32and HD 36 combine to switch the magnetic orientation of the memoryelement 12.

Large bit and digit line currents are undesirable because memory arraypower consumption is a serious limiting factor in MRAM applications.High bit and digit currents require larger bit and digit lines and writecircuits to handle the high currents. This may result in larger, moreexpensive MRAM devices. However, there is an ever-increasing demand forsmaller memory devices. While smaller device size may be achievedthrough techniques such as patterning smaller memory elements, a smallermemory element increases the shape component of the anisotropyassociated with the memory element. As the anisotropy increases, theamount of current necessary to alter the magnetization direction alsoincreases.

Accordingly, it is desirable to provide a low power MRAM memory elementthat requires reduced or minimized current to alter the magneticdirection of the element. In addition, it is desirable to provide anMRAM device that requires low power for programming. Furthermore, otherdesirable features and characteristics of the present invention willbecome apparent from the subsequent detailed description of theinvention and the appended claims, taken in conjunction with theaccompanying drawings and this background of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 illustrates a conventional memory element array;

FIG. 2 illustrates magnetic fields generated at a memory element of aconventional memory element array;

FIG. 3 is a cross-sectional view of a memory element in accordance withan exemplary embodiment of the present invention;

FIG. 4 is a plan view of the memory element of FIG. 3 illustratingmagnetic fields generated at the memory element;

FIG. 5 is a graphical illustration of a programming window of the memoryelement of FIG. 3;

FIG. 6 is a cross-sectional view of a memory element in accordance withanother exemplary embodiment of the present invention;

FIG. 7 is a graphical illustration of the relationship between ananti-ferromagnetic coupling saturation field of an anti-ferromagneticcoupling material and the thickness of the anti-ferromagnetic couplingmaterial;

FIG. 8 is a cross-sectional view of a memory element in accordance witha further exemplary embodiment of the present invention;

FIG. 9 is a schematic illustration of a memory element array havingmemory elements, shown in phantom, in accordance with an exemplaryembodiment of the present invention;

FIG. 10 is a schematic illustration of a memory element having anelliptical shape; and

FIG. 11 is a schematic illustration of a memory element having arectangular shape.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Turning now to FIG. 3, in accordance with an exemplary embodiment of thepresent invention, a simplified sectional view of an MRAM array 100comprises a scalable magnetoresistive memory element 102. In thisillustration, only a single magnetoresistive memory element 102 is shownfor simplicity in describing the embodiments of the present invention,but it will be understood that MRAM array 100 may consist of a number ofmagnetoresistive memory elements 102.

Magnetoresistive memory element 102 is sandwiched between a bit line 122and a digit line 120. Bit line 122 and digit line 120 include conductivematerial such that a current can be passed therethrough. In thisillustration, bit line 122 is positioned on top of magnetoresistivememory element 102 and digit line 120 is positioned on the bottom ofmagnetoresistive memory element 102, and is directed at a 90-degreeangle to bit line 122. While bit line 122 and digit line 120 areillustrated with physical contact to memory element 102, it will beunderstood that the various embodiments of the present invention are notso limited and bit line 122 and/or digit line 120 may be physicallyseparated from memory element 102. In addition, while bit line 122 isillustrated positioned above digit line 120, it will be understood thatthe reverse positioning of digit line 120 and bit line 122 may beutilized.

Magnetoresistive memory element 102 comprises a first magnetic region104, a second magnetic region 106, and a tunnel barrier 108 disposedbetween first magnetic region 104 and second magnetic region 106. In oneembodiment of the invention, magnetic region 104 includes a syntheticanti-ferromagnetic (SAF) structure 110, a structure having ananti-ferromagnetic coupling spacer layer 134 sandwiched between twoferromagnetic portions 130 and 132. Further, second magnetic region 106may have an SAF structure 112, which has an anti-ferromagnetic couplingspacer layer 144 disposed between two ferromagnetic portions 140 and142. However, it will be appreciated that second magnetic region 106 mayhave any structure suitable for forming an operable memory element 102.

Ferromagnetic portions 130 and 132 each have a magnetic moment vector150 and 152, respectively, that are usually held anti-parallel by theanti-ferromagnetic coupling spacer layer 134. Magnetic region 104 has aresultant magnetic moment vector 154 and magnetic region 106 has aresultant magnetic moment vector 156. Resultant magnetic moment vectors154 and 156 are oriented along an anisotropy easy-axis in a directionthat is at an angle from bit line 122 and digit line 120. In oneembodiment of the invention, the resultant magnetic moment vectors 154and 156 are oriented at angle in the range of about 30 degrees to about60 degrees from bit line 122 or digit line 120. In a preferredembodiment of the invention, the resultant magnetic moment vectors 154and 156 are oriented at an angle of about 45 degrees from bit line 122and digit line 120. Further, magnetic region 104 is a free ferromagneticregion, meaning that resultant magnetic moment vector 154 is free torotate in the presence of an applied magnetic field. Magnetic region 106is a pinned ferromagnetic region, meaning that resultant magnetic momentvector 156 is not free to rotate in the presence of a moderate appliedmagnetic field and is used as the reference layer.

The magnetic moment vectors 150 and 152 of the two ferromagneticportions 130 and 132 can have different thicknesses or material toprovide resultant magnetic moment 154 given by ΔM=M₂−M₁. In a preferredembodiment of the invention, the SAF structure 110 will be substantiallybalanced; that is, ΔM is less than 15 percent of the average of M2−M1(otherwise simply stated as “the imbalance is less than 15 percent) andis more preferably as near to zero as can be economically fabricated inproduction lots.

During fabrication of MRAM array 100, each succeeding layer, discussedin more detail below, is deposited or otherwise formed in sequence andeach memory element 102 may be defined by selective deposition,photolithography processing, etching, etc. using any of the techniquesknown in the semiconductor industry. During deposition of at least theferromagnetic portions 130 and 132, a magnetic field is provided to seta preferred anisotropy easy-axis (induced intrinsic anisotropy). Theprovided magnetic field creates a preferred anisotropy easy-axis formagnetic moment vectors 150 and 152. As described in more detail below,in addition to intrinsic anisotropy, memory elements having aspectratios greater than one may have a shape anisotropy that defines an easyaxis that is parallel to a long axis of the memory element. This easyaxis may also be selected to be at about a 30 to 60 degree angle,preferably at about a 45-degree angle, between the bit line 122 and thedigit line 120.

FIG. 4 illustrates a simplified plan view of MRAM array 100 inaccordance with an embodiment of the present invention. To simplify thedescription of magnetoresistive memory element 102, all directions willbe referenced to an x- and y-coordinate system 160 as shown. To furthersimplify the description, only the magnetic moment vectors of region 104are illustrated since they will be switched. As shown, a bit current IB170 is defined as being positive if flowing in a positive x-directionand a digit current ID 172 is defined as being positive if flowing in apositive y-direction. A positive bit current IB 170 passing through bitline 122 results in a circumferential bit magnetic field, HB 174, and apositive digit current ID 172 will induce a circumferential digitmagnetic field HD 176. The magnetic fields HB 174 and HD 176 combine toswitch the magnetic orientation of first magnetic region 104 of memoryelement 102.

FIG. 5 is a graphical representation 200 of a programming region orwindow, in terms of magnetic field HB 174 and magnetic field HD 176,within which first magnetic region 104 may be switched reliably. In MRAMarray 100, an individual memory element is programmed by flowing currentthrough the bit line and the digit line proximate to the individualmemory element. Information is stored by selectively switching themagnetic moment direction of first magnetic region 104 of the individualmemory element 102. The memory element state is programmed to a “1” or“0” depending on the previous state of the bit; that is, a “1” isswitched to a “0” or a “0” to a “1”. All other memory elements 102 areexposed only to fields from a single line (½-selected memory elements),or no lines. A memory element is switched reliably when the magneticregion 104 of the memory element switches deterministically between a“0” state and a “1” state upon application or withdrawal of a magneticfield. A memory element that switches somewhat randomly between a “0”state and a “1” state upon application or withdrawal of a magnetic fielddoes not provide reliable or desirable switching.

Due to process and material variations, an array of memory elements 102has a distribution of switching fields with a mean value

Hsw

and a standard deviation σsw. Typically, the array of memory elements102 is required to meet a predetermined switching or programming errorrate. Accordingly, to program the memory elements 102 in MRAM array 100with approximately the same currents, the applied field produced fromthe currents preferably is larger than the mean switching field

Hsw

by no less than approximately Nσsw, where N is a positive number largeenough to ensure the actual switching error rate does not exceed thepredetermined programming error rate, and is typically greater than orequal to 6 for memories whose size are about 1 Mbit or larger.

In addition, there is a maximum saturation field HSAT that can beapplied to a selected memory element to ensure reliable switching. Thefield HSAT corresponds to that field which, when applied to magneticregion 104, causes magnetic moment vector 150 and 152 to be alignedapproximately parallel. Therefore, HSAT is known as the saturation fieldof the SAF structure in region 104 and is a measure of theanti-ferromagnetic coupling between layers 130 and 132. Also due toprocess and material variations, an array of memory elements 102 has adistribution of saturation fields with a mean value

HSAT

and a standard deviation □sat. Therefore, the applied field preferablyis kept less than approximately

HSAT

−N□sat or the selected memory element will not be programmed reliably.

Thus, for reliable programming that meets a predetermined switchingerror rate or has an error rate below the predetermined switching errorrate, there is an operating window 202 for an applied magnetic field Hthat results from programming fields HB 174 and HD 176. The magnitude ofthe operating window, Hwin, along the dotted line shown in FIG. 5 isrepresented approximately by the equation Hwin≈(

Hsat

−

sat)−(

Hsw

+

□sw). Inside this window 202, substantially all the memory elements canbe programmed without error. Outside this window, the memory elementscannot be programmed or cannot be programmed without possible errors.For example, the region 204 of graphical representation 200 is thatregion where a magnetic field H applied to memory element 102 by bitcurrent IB 170 and digit current ID 172 is greater than HSAT and firstmagnetic region 104 of magnetoresistive memory element 102 does notswitch reliably between both the “1” and “0” states. The region 206 ofgraphical representation 200 is that region where the applied field H isless than the switching field HSW and first magnetic region 104 ofmagnetoresistive memory element 102 does not switch.

The magnetic switching field for writing to memory element 102 isrepresented by the equation:H _(SW) ≅√{square root over (H_(k)H_(SAT))},

where Hk is the total anisotropy of first magnetic region 104 and HSAT,as described above, is the anti-ferromagnetic coupling saturation field,that is, HSAT is the maximum magnetic field at which first magneticregion 104 of magnetoresistive memory element 102 will switch reliably.Hk may be represented by the equation:Hk(total)=Hk(intrinsic)+Hk(shape),

where Hk(intrinsic) is the intrinsic anisotropy of the materialcomprising magnetic region 104 and Hk(shape) is the anisotropy due tothe shape of magnetic region 104. Similarly, HSAT may be represented bythe equation:HSAT(total)=HSAT(intrinsic)+HSAT(shape).

In this equation, HSAT(intrinsic) is the magnetic field at which themagnetic layers of first magnetic region 104 are substantially parallelto each other when formed as continuous films and HSAT(shape) representsthe magnetostatic coupling of the magnetic layers of magnetic region 104as a result of the shape of the magnetic region 104.

Accordingly, to reduce the power required by magnetoresistive memoryelement 102, that is, to reduce or minimize the current required toswitch first magnetic region 104 of magnetoresistive memory element 102,HSW of magnetic region 104 may be reduced or minimized. To minimize HSW,Hk(total) or HSAT(total) or both may be reduced or minimized. Thus, inaccordance with an embodiment of the invention, Hk(intrinsic),Hk(shape), HSAT(intrinsic), or HSAT(shape), or any combination thereof,may be reduced or minimized.

Referring again to FIG. 3, in accordance with an exemplary embodiment ofthe present invention, to reduce or minimize the current required toswitch first magnetic region 104, and thus reduce the power required bymemory element 102, ferromagnetic portions 130 and 132 may be fabricatedsuch that magnetic region 104 has a low Hk(total) value. However, in apreferred embodiment of the invention, magnetic region 104 may not havean Hk(total) value that is so low that magnetic region 104 and, hence,magnetoresistive memory element 102, are thermally unstable andvolatile. Thermal instability refers to the switching of the memorystate due to thermal fluctuations in the magnetic layers 130 and 132.The energy barrier Eb to thermal fluctuations for first magnetic region104 is represented approximately by the equation Eb=MS×V×Hk, where MS isthe saturation magnetization of the magnetic material in layers 130 and132, V is the total volume (area×thickness) of layers 130 and 132, andHk is as defined above. In one embodiment of the invention, Hk(total)has a value of less than about 15 Oe-microns divided by region width,where the “region width” is the dimension (in microns) of the firstmagnetic region 104 that is orthogonal to the longitudinal axis of thefirst magnetic region 104 and the thickness of the first magnetic region104. In a preferred embodiment of the present invention, Hk(total) has avalue in the range of from about 10 Oe-microns÷region width (in microns)to about 15 Oe-microns region width (in microns).

In one embodiment of the invention, to reduce Hk(total) and, hence, toreduce the power requirements of memory element 102, ferromagneticportions 130 and 132 may be formed of one or more layers of material ormaterials having a low Hk(intrinsic) value. As used herein, the term lowHk(intrinsic) value means an Hk(intrinsic) value of less than or equalto about 10 Oe. Examples of materials that have a low Hk(intrinsic)value that is suitable for forming ferromagnetic portions 130 and 132 ofmagnetic region 104 but that does not render magnetic region 104thermally unstable include nickel (Ni), iron (Fe), cobalt (Co), oralloys of Ni, alloys of Fe, or alloys of Co, such as NiFeB, NiFeMb,NiFeTa, NiFeCo, and the like. Ferromagnetic portions 130 and 132 may beformed of the same material or may be formed of different materialshaving a low Hk(intrinsic) value.

In accordance with another embodiment of the present invention, toreduce the power requirements of memory element 102, magnetic region 104may be fabricated utilizing a material or materials that produce a lowHk(shape) value to form ferromagnetic portions 130 and 132. Again,however, it is preferred that the material that forms magnetic region104 may not produce an Hk(total) value that is so low that magneticregion 104 and, hence, magnetoresistive memory element 102, arethermally unstable and volatile. As discussed above, materials producinga low Hk(shape) value for a given memory element shape include materialshaving a low saturation magnetization MS. As used herein, the term “lowsaturation magnetization”, or “low magnetization”, refers to thosematerials having a magnetization that is less than or equal to themagnetization of Ni80Fe20. Ni80Fe20 has a magnetization approximatelyequal to 800 kA/m and a saturation flux density of approximately 1Tesla. As the magnetization of the material(s) that form ferromagneticportions 130 and 132 also directly affect the magnetostatic coupling ofthe layers, the use of a low magnetization material(s) for ferromagneticportions 130 and 132 also serves to reduce or minimize HSAT(shape).Accordingly, the lower the magnetization of the material(s) of portions130 and 132 is, the lower the Hk(shape) and the HSAT(shape) values are.Low magnetization materials suitable for forming ferromagnetic portions130 and 132 comprise Ni80Fe20 and alloys of Ni, alloys of Fe, or alloysof Co, such as, for example, NiFeB, NiFeMb, NiFeTa, and NiFeCo. Again,ferromagnetic portions 130 and 132 may be formed of the same ordifferent low magnetization materials.

The doping of Ni80Fe20 with materials such as molybdenum, tantalum,boron, and the like also may result in a material with a lowHk(intrinsic) value and a magnetization less than those of Ni80Fe20,thus facilitating fabrication of a low power memory element 102.However, doping with such materials also may decrease themagnetoresistance through tunnel barrier 108, and thus decrease theperformance of memory element 102. Although the spin polarization of thetunneling electrons determines the magnetoresistance, low magnetizationmaterials typically also have low spin polarization. Accordingly, in onealternative embodiment of the invention, as illustrated in FIG. 6, amagnetoresistive memory element 250 may have a ferromagnetic portion 132that comprises two materials, a first material 252 with a lowmagnetization that reduces the value of Hk(shape) of magnetic region 104and a second material 254, disposed close to the tunnel barrier 108,with a high polarization that compensates for the decrease in themagnetoresistance due to the first material 252. As used herein, theterm “high polarization material” is any material having a spinpolarization that is greater than or equal to Ni80Fe20. Second material254 may comprise material such as, for example, Co, Fe, and CoFe and mayalso comprise Ni80Fe20 when the first material 252 has a magnetizationlower than Ni80Fe20. In a preferred embodiment of the invention, firstmaterial 252 and/or second material 254 comprise materials that alsohave a low Hk(intrinsic), as described above. As first magnetic region104 is preferably a moment-balanced SAF structure, in one embodiment ofthe invention, ferromagnetic portion 130 has a thickness such that themagnetic moments of ferromagnetic portions 132 and 130 have the samemagnitude. In another embodiment of the invention, ferromagnetic portion130 also comprises first material 252 and second material 254.

The Hk(shape) of a single magnetic layer is approximately proportionalto N_(d)×M_(s)×t/w where Nd is a demagnetizing factor that increaseswith aspect ratio, t is the thickness of the layer, and w is the regionwidth. This formula also applies for the layers in the SAF structure offirst magnetic region 104. Although the SAF structure of first magneticregion 104 does reduce Hk(shape) compared to a single film of comparablethickness 2×t, the Hk(shape) is still finite due to asymmetry in theswitching process. The magnetic layers are not perfectly antiparallelduring switching, so that each layer's magnetostatic fields (thatproduce Hk(shape)) do not perfectly cancel one another.

In another embodiment of the present invention, magnetic region 104 maybe fabricated with the minimum possible thickness t for ferromagneticlayers 130 and 132. As discussed above, a thinner thickness t willresult in a smaller Hk(shape) and Hsat(shape) since the magnetostaticfields that produce Hk(shape) and Hsat(shape) are proportional tothickness. The minimum thickness possible is limited by the requirementof thermal stability. Note that by reducing t, both Hk(shape) and totalvolume V of layers 130 and 132 are reduced for the bit, so that theenergy barrier is reduced by approximately t2. In addition to thethermal stability requirement, the minimum thickness is also limited bythe ability to grow a high quality continuous magnetic film on top ofthe tunnel barrier. In one embodiment of the invention, the optimumminimum thickness t of layers 130 and 132 is within a range of fromabout 3.5 nm to about 5 nm.

Referring again to FIG. 3, in accordance with a further embodiment ofthe present invention, to reduce the power requirements of memoryelement 102, first magnetic region 104 also may be fabricated to have alow Hk(shape) value by forming it in a shape having a low aspect ratio.In one embodiment of the invention, first magnetic region 104 has alength preferably measured along a long axis of region 104, and a widthmeasured orthogonal to the length, and a length/width ratio in a rangeof about 1 to about 3 for a non-circular plan. For example, asillustrated in FIG. 10, in one embodiment of the invention, a memoryelement 400, which may be the same as memory element 102, may have afirst magnetic region 104 of an elliptical shape with a length 402 andwidth 404 and with a length/width ratio of about 1 to about 3. Inanother embodiment of the invention, as illustrated in FIG. 11, a memoryelement 410, which may be the same as memory element 102, may have afirst magnetic region 104 of a rectangular shape with a length 412 andwidth 414 and having a length/width ratio of about 1 to about 3.Alternatively, the first magnetic region 104 of a memory element may becircular in shape (length/width ratio of 1) to minimize the contributionto the switching field from shape anisotropy Hk(shape) and also becauseit is easier to use photolithographic processing to scale the device tosmaller dimensions laterally. However, it will be understood that firstmagnetic region 104 can have any other suitable shape, such as square ordiamond. In a preferred embodiment of the invention, first magneticregion 104 has a length/width ratio in a range of about 2 to about 2.5.

In accordance with yet another embodiment of the present invention,magnetic region 104 may be fabricated to reduce or minimize HSAT(total)to reduce the power requirements of memory element 102. Again, however,as discussed above with reference to FIG. 5, magnetic region 104 may nothave an HSAT(total) value that is so low that there is no operableprogramming window. In other words, while HSAT(total) may be reduced orminimized, its value preferably is such that the programming windowoperable for switching magnetic region 104 can be defined as above bythe equation Hwin≈(

H_(SAT)

−

□sat)−(

Hsw

+

□sw), where Hwin is a magnetic field applied to magnetoresistive memoryelement 102 by currents ID and IB to switch magnetic region 104. In oneembodiment of the invention, HSAT(total) has a value in the range offrom about 150 Oe to about 350 Oe. In a preferred embodiment,HSAT(total) has a value less than or equal to approximately 180/w0.5(Oe), where w is the region width of magnetic region 104, as previouslydescribed.

At present memory element dimensions in the range of 0.5 to 1 micron,the dominant contribution to HSAT(total) is from HSAT(intrinsic).HSAT(intrinsic) is determined by the anti-ferromagnetic couplingmaterial that comprise anti-ferromagnetic coupling spacer layer 134 andits thickness. Generally, anti-ferromagnetic coupling spacer layer 134comprises one of the elements ruthenium, osmium, rhenium, chromium,rhodium, copper, or combinations thereof. Preferably, anti-ferromagneticcoupling spacer layer 134 comprises ruthenium. In one embodiment of thepresent invention, HSAT(intrinsic), and hence HSAT(total), may bereduced or minimized by fabricating anti-ferromagnetic coupling spacerlayer 134 with a thickness such that magnetic region 104 comprises asecond order SAF. FIG. 7 is a graph that illustrates a typicalrelationship between the value of HSAT(intrinsic) and the thickness ofan anti-ferromagnetic coupling material, such as ruthenium, that may beused to form anti-ferromagnetic coupling spacer layer 134. As shown inFIG. 7, the anti-ferromagnetic coupling material operates as ananti-ferromagnetic coupling spacer layer 134 at a first peak or firstrange of thicknesses 280. At first peak 280, the anti-ferromagneticcoupling spacer layer 134 forms a first order SAF with ferromagneticlayers 130 and 132 of FIG. 3. The anti-ferromagnetic coupling materialalso may operate as an anti-ferromagnetic coupling spacer layer 134 at asecond peak or range of thicknesses 282, thus forming a second order SAFwith ferromagnetic layers 130 and 132. As illustrated in FIG. 7, thevalues of HSAT(intrinsic) are relatively higher at the first peak 280than at the second peak 282. Thus, by forming magnetic region 104 as asecond order SAF, that is, with an anti-ferromagnetic coupling spacerlayer 134 having a thickness within the range of thicknesses of thesecond peak 282, HSAT(total) may be reduced or minimized, thus reducingor minimizing HSW. In addition, the second peak is much flatter as afunction of spacer layer thickness compared to the first order peak, sothat the spacer layer thickness can vary over a wider range and stillsupply an HSAT(intrinsic) of nominally the same magnitude. HSATinsensitivity to spacer layer thickness may be desirable for robust andreproducible manufacturing.

As described above, while it is preferable that HSAT(total) be minimizedto lower the power requirements of magnetoresistive memory element 102,HSAT(total) preferably is large enough that there exists an operableprogramming window for programming memory element 102. Thus, while itmay be desirable to fabricate magnetic region 104 as a second order SAF,HSAT(total) may be too low to provide a satisfactory programming windowfor memory element 102. As illustrated by the third peak 284 in FIG. 7,the presence of a material that produces higher anti-ferromagneticexchange coupling, such as a material comprising Co, Fe, or CoFe,disposed proximate to a surface of anti-ferromagnetic coupling spacerlayer 134 may increase HSAT(intrinsic) to acceptable values.Accordingly, referring to FIG. 8, in another embodiment of the presentinvention, a magnetoresistive memory element 300 may comprise a firstinterface layer 302 disposed at a first surface of anti-ferromagneticcoupling spacer layer 134 and/or a second interface layer 304 disposedat a second surface of anti-ferromagnetic coupling spacer layer 134.Materials suitable for forming interface layers 302 and 304 comprisematerials such as Co, Fe, CoFe, and alloys of Co or alloys of Fe, suchas, for example, CoFeTa or CoFeB.

Referring again to FIG. 7, in another embodiment of the invention,magnetic region 104 may be fabricated as a first order SAF, that is,with an anti-ferromagnetic coupling spacer layer 134 having a thicknesswithin the range of thicknesses of the first peak 280. Preferably,however, anti-ferromagnetic coupling spacer layer 134 has a thicknessthat is larger than a thickness tmax that results in a maximumHSAT(intrinsic). In this regard, HSAT(intrinsic) may be optimized alongfirst peak 280 to reduce the power requirements of memory element 102but also to provide a suitable programming window within which memoryelement 102 may be switched.

In yet another embodiment of the invention, when magnetic region 104 isfabricated as a first order SAF, HSAT(intrinsic) may be furtheroptimized by utilizing interface layers 302 and/or 304, as illustratedin FIG. 8. For practical reasons, it may be desirable to fabricatemagnetic region 104 with an anti-ferromagnetic coupling spacer layerthickness that exhibits an HSAT(intrinsic) that is approximately equalto or below a predetermined HSAT(intrinsic). For example, it may bedesirable to form the anti-ferromagnetic coupling spacer layer with athickness such that any deviations of thickness during processing do notsignificantly affect the value of HSAT(intrinsic). In other words, itmay be desirable to form the anti-ferromagnetic coupling spacer layerwith a thickness that is at a flatter or more stable region of the firstpeak 280. However, at this thickness, HSAT(intrinsic) may be below adesired HSAT(intrinsic). Thus, interface layers 302 and/or 304, asillustrated in FIG. 8, may be utilized to increase the HSAT(intrinsic)to the desired value.

HSW also may be reduced or minimized, thus reducing the powerrequirements of memory element 102, by reducing or minimizingHSAT(shape). As described above, in one embodiment of the presentinvention, HSAT(shape) may be reduced or minimized by fabricatingmagnetic layers 130 and 132 from a low magnetization material. Also asdescribed above, in another embodiment of the present invention,HSAT(shape) may be reduced or minimized by fabricating magnetic layers130 and 132 with a minimum thickness t. In another exemplary embodimentof the present invention, HSAT(shape) also may be reduced by fabricatingmemory element 102 with a shape having one or more substantially sharpor pointed ends along the anisotropy axis that exhibit magnetostaticcoupling of ferromagnetic layers 130 and 132 that is lower than themagnetostatic coupling of layers 130 and 132 of a memory element 102having a shape with substantially rounded ends, such as acircular-shaped memory element 102. For example, as illustrated in FIG.9, memory element 102 may be formed in the shape of an ellipse thatcomprises substantially sharp or pointed ends 320 along a longitudinalaxis 322 of the memory element. A memory element 102 having this shapewill exhibit less magnetostatic coupling, and hence a lower HSAT(shape)value, than a comparable memory element 102 having a circular shape oran elliptical shape with substantially rounded ends. It will beappreciated, however, that memory element 102 may be fabricated with avariety of other shapes, such as a diamond shape, that will exhibitreduced magnetostatic coupling and hence a reduced or minimizedHSAT(shape).

Accordingly, magnetoresistive random access memory elements that requirelower power for programming in accordance with the present inventionhave been described. The power requirements for programming the memoryelements are related to the magnetic switching field HSW represented bythe equation H_(SW)≅√{square root over (H_(k)H_(SAT))}. The embodimentsof the present invention provide methods and structures for reducingand/or minimizing Hk and HSAT. While at least one exemplary embodimenthas been presented in the foregoing detailed description of theinvention, it should be appreciated that a vast number of variationsexist. It should also be appreciated that the exemplary embodiment orexemplary embodiments are only examples, and are not intended to limitthe scope, applicability, or configuration of the invention in any way.Rather, the foregoing detailed description will provide those skilled inthe art with a convenient road map for implementing an exemplaryembodiment of the invention, it being understood that various changesmay be made in the function and arrangement of elements described in anexemplary embodiment without departing from the scope of the inventionas set forth in the appended claims.

1. A magnetoresistive random access device having an array of memoryelements, each memory element comprising: a fixed magnetic portion, atunnel barrier portion disposed proximate to said fixed magneticportion; and a free SAF structure disposed proximate to said tunnelbarrier portion, wherein: the array of memory elements has a finitemagnetic field programming window H_(win) represented by the equationH_(win)≈(

Hsat

−

σ_(sat))−(

Hsw

+

σ_(sw));

Hsw

is a mean switching field for said array;

Hsat

is a mean saturation field for said array; Hsw for said each memoryelement is represented by the equation H_(SW)≅√{square root over(H_(k)H_(SAT))}, wherein H_(k) represents a total anisotropy field ofsaid free SAF structure of said each memory element and H_(SAT)represents a total anti-ferromagnetic coupling saturation field for saidfree SAF structure of said each memory element; N is an integer greaterthan or equal to 1; σ_(sw) is a standard deviation for

Hsw

; and σ_(sat) is a standard deviation for

H_(SAT)

, and wherein said free SAF structure is configured to have H_(k),H_(SAT), and N values such that the array of memory elements isthermally stable and requires current to operate that is below apredetermined current value.
 2. The magnetoresistive random accessdevice of claim 1, wherein H_(k) of said free SAF structure of said eachmemory element of said array has a value that is no greater than 15Oe-microns divided by a width of said free SAF structure, wherein saidwidth is a dimension (in microns) of said free SAF structure that isorthogonal to a longitudinal axis of said free SAF structure.
 3. Themagnetoresistive random access device of claim 1, wherein said free SAFstructure comprises two magnetic portions, each of said two magneticportions comprising a layer of low magnetization material.
 4. Themagnetoresistive random access device of claim 3, each of said at leasttwo magnetic portions comprising a material selected from the groupconsisting of Ni, Fe, Co, alloys of Ni, alloys of Fe, and alloys of Co.5. The magnetoresistive random access device of claim 3, wherein saidlow magnetization material is doped with at least one material selectedfrom the group consisting of molybdenum, tantalum, and boron.
 6. Themagnetoresistive random access device of claim 3, at least one of saidtwo magnetic portions further comprising a layer of high spinpolarization material.
 7. The magnetoresistive random access device ofclaim 1, wherein said free SAF structure comprises two magneticportions, each of said two magnetic portions having a thickness nogreater than about 5 nm.
 8. The magnetoresistive random access device ofclaim 1, wherein said free SAF structure has a length and a width and alength/width ratio in the range of from about 1 to about
 3. 9. Themagnetoresistive random access device of claim 8, said free SAFstructure having a length/width ratio in the range of from about 2 toabout 2.5.
 10. The magnetoresistive random access device of claim 1,said free SAF structure having a value of HSAT in the range of fromabout 150 Oe to about 350 Oe.
 11. The magnetoresistive random accessdevice of claim 1, said free SAF structure is configured to have a valueof H_(SAT) less than approximately 180/w^(0.5) (Oe), where w is thewidth (in microns) of said free SAF structure.
 12. The magnetoresistiverandom access device of claim 1, wherein said free SAF structure is asecond order SAF structure.
 13. The magnetoresistive random accessdevice of claim 12, wherein said free SAF structure comprises twomagnetic portions separated by an anti-ferromagnetic coupling spacerlayer and a layer of material that produces a higher anti-ferromagneticexchange coupling than said anti-ferromagnetic coupling spacer layeralone.
 14. The magnetoresistive random access device of claim 1, whereinsaid free SAF structure comprises two magnetic portions separated by ananti-ferromagnetic coupling spacer layer, wherein a thickness of saidanti-ferromagnetic coupling spacer layer is such that said free SAFstructure is a first order SAF structure and wherein said thickness ofsaid anti-ferromagnetic coupling spacer layer is greater than athickness at which an anti-ferromagnetic coupling saturation fieldH_(SAT) of said anti-ferromagnetic coupling material is a maximum. 15.The magnetoresistive random access device of claim 1, wherein said freeSAF structure has an anisotropy axis and has a shape with at least onesubstantially pointed end disposed substantially along said anisotropyaxis.